AMOLED display is one of the hotspots in the current panel display technology. As compared with a liquid crystal display, an OLED presents a number of advantages, such as lower energy consumption, lower production cost, self-luminescence, a wider viewing angle, and a faster response speed. The design for the pixel unit driving circuit is a core of AMOLED display and is important to be researched.
A lighting principle for the AMOLED is that an OLED is driven by current generated by drive transistor (Thin Film Transistor) in a saturation state. Different from TFT-LCD (Thin Film Transistor Liquid Crystal Display) the brightness of which is controlled by a stable voltage, the OLED is driven by current, which needs a stable current to control lighting. When identical grey scale voltages are input, different critical voltages generate different driving current, which leads to inconsistency of the currents.
FIG. 1 shows a conventional OLED pixel unit driving circuit. As shown in FIG. 1, the conventional AMOLED pixel unit driving circuit utilizes a pixel unit driving circuit in a 2T1C configuration. The circuit comprises one drive TFT (T2), one switch TFT (T1) and a storage capacitance Cs. When the scan line selects (i.e. scans) one line to be on and the Vscan is a low level signal, the transistor T1 is turned on and the data signal Vdata is written into the storage capacitance Cs. When the scanning of the line is ended and the Vscan becomes a high level signal, the transistor T1 is turned off and the gate voltage stored on the storage capacitance Cs drives the transistor T2 so that the transistor T2 generates current to drive the OLED to maintain that the OLED continuously irradiate during one frame. The current for the drive TFT (T2) in a saturation state is shown by an equation of IOLED=K(VGS−Vth)2.
Due to reasons such as process and aging of devices, a threshold voltage (Vth) for the drive TFT of the respective pixels drift, which leads to that the current flowing through each of the OLED pixels varies with the change of Vth. Moreover, since there are non homogeneity among the threshold voltages of the drive TFTs for the respective pixels, it directly results in that the current flowing through the OLED of each of the pixels change so that the displayed brightness is uneven and the display effect of the whole image is influenced.